A remarkable seismic phenomenon at Erebus is the ubiquitous association of lave lake Strombolian eruptions with repeatable very long period (VLP) oscillations that persist for several minutes. VLP signals presage eruptions by up to several seconds, indicating that they are initially excited by the upward rise of gas slugs in the shallowest portion of the conduit system. The Erebus VLP spectrum is observed to contain numerous peaks, dominated by three strong harmonics near 21 s, 11 s, and 8 s (see figure above). VLP signals are common on active volcanoes and usual arise due to pressurization and reaction forces associated with the movement of magma and/or gas.

At Erebus, the VLP signal is initially driven by pressure changes in the irregular conduit as Strombolian gas slugs rise under buoyancy forces, followed by oscillatory pressurization and single forces resulting from by the movement of magma from deeper reservoirs to refill the lava lake.

Recent moment tensor inversion suggests that the centroid depth for these forces is around 400 m below the lava lake, which may indicate the approximate position of a shallow magma chamber. The accompanying figure shows a spectrogram constructed from a stack of 35 1999-2000 VLP signals recorded at station E1S. VLP signals have also recently been observed on many other active volcanoes, such as Stromboli, Kilauea, and Popocatepetl, and offer a unique geophysical view into the dynamics and geometry of volcanic plumbing and moving magma.